The present invention relates to testing of integrated circuits and, more specifically, to a method for testing the impedance of the power supply wiring of circuit boards and integrated circuits and to a circuit for use therewith.
The number of pins for new integrated circuits (ICs) has been increasing for many years, and is likely to continue increasing. Most ICs have fewer than 100 pins, but many have more than 1000. For these larger ICs, between one third and two thirds of the pins can be power supply pins. The large proportion of power supply pins is needed to minimize the inductance and resistance between the off-chip power supply voltage and the on-chip circuitry. When logic gates switch, especially CMOS logic gates, their logic outputs can change state in less than a nanosecond and this can cause a transient voltage of similar duration on the power supply voltage rails due to the non-zero resistance and inductance in these rails. The transient voltage amplitude can exceed a third of the dc power supply voltage, but is typically less than 10% of this voltage. The transient voltage, if excessive, can increase or decrease the signal propagation delay through logic gates in the integrated circuit, which can cause logic errors. For this reason, IC designers ensure that sufficient power supply pins are used, while trying to avoid using too many pins (due to cost and area).
When an IC is soldered to a circuit board, it is possible that one or more of the many power supply pins is not soldered. This type of manufacturing defect is very difficult to detect electrically because the same board wire is connected in parallel to many of the IC""s power pins, and the pins are also connected together within the chip. If the typically milliohm resistance and nanohenry inductance between an unconnected power pin and the board wire is measured, it will be almost indistinguishable from the resistance and inductance for a correctly connected pin. In any case, it is not practical in a production test environment to probe the hundreds of power pins of a high pin-count IC.
In a paper entitled, xe2x80x9cOpens Board Test Coverage: When is 99% Really 40%?xe2x80x9d, published in the 1996 International Test Conference Proceedings, the problem of unconnected power supply pins is described in detail, and solutions proposed include X-ray laminography and optical inspection to detect missing solder on pins. The equipment costs hundreds of thousands of U.S. dollars, requires very specialized programming, a long test time relative to other tests (xe2x80x9cbetween 30 and 40 joints/secondxe2x80x9d), and does not detect any on-chip connection defects.
In a paper entitled, xe2x80x9cPower Pin Testing: Making the Test Coverage Completexe2x80x9d, published in the 2000 International Test Conference Proceedings, a sensitive on-chip analog comparator is used to monitor the dc voltage drop across a segment of the on-chip power supply wiring near each power supply pin. This requires a low impedance load circuit to be activated in the IC to cause excess current to flow, the technique can detect excessive resistance only, and the IC area of the circuitry is relatively large (comparable to the area of a bond pad): 0.6% of the IC area for a 256 pin IC having 40 power pins, according to the referenced paper. The circuit requires several chip-wide signals to control the comparators, and the comparators require about 5 microseconds to perform a measurement (and hence cannot respond to nanosecond transient voltages).
In prior art for detecting voltage deviations on power supply rails, the intent is to detect when the power supply voltage decreases below some threshold voltage, below which, correct function of a main circuit is uncertain. FIG. 1A shows a schematic of a circuit 10 for detecting supply voltage deviations. The output 12 of the comparator 14 is typically connected to the reset of the main circuit, or to the reset of a flip-flop which is connected to the power-down input of the main circuit. The comparator compares the power rail voltage VDD divided by some number, such as two for example, as generated using a resistive divider formed by resistors 16 and 18, to a constant voltage derived across a zener diode 19. Circuit 10 is typically used to detect both rapid power loss (xe2x80x9cblackoutxe2x80x9d) and a slow decrease (xe2x80x9cbrownoutxe2x80x9d) of the power supply voltage. Faster decreases (xe2x80x9ctransientsxe2x80x9d) are also detectable; however, comparators cannot typically respond to transients faster than 10 nanoseconds, and this is usually intentional so that noise xe2x80x9cglitchesxe2x80x9d on the power rail do not cause inadvertent reset of the main circuit powered by the power rail.
There is a need for a low cost test method for detecting the increase in impedance (resistance or inductance) caused by a missing power supply pin connection or any other defect, including poor design, that results in excessive transient voltages typically lasting less than 10 nanoseconds on power supply rails; these transients can cause incorrect logic signal propagation delays or incorrect logic operation.
The present invention provides a low cost test method and test circuit for detecting a missing power supply pin connection or any other defect that results in excessive transient voltages on power supply rails of a circuit.
The circuit includes a sensor logic gate whose input is connected to a dc voltage that is made independent of transient signals on the power supply rails of the sensor logic gate. An output of the sensor logic gate is connected to an input of a memory element, such as a latch or flip-flop. A stimulus logic gate, which causes a transient voltage and current on the power supply rails when it changes state, is connected to the same power supply rails as the sensor logic gate. A signal transition is applied to the input of the stimulus logic state and the state of the memory element is inspected. If the memory element changed state, there is a power supply defect associated with the power supply rails.
A preferred embodiment of the invention described herein uses a single, embedded digital latch or flip-flop per power rail point to be tested, and uses a single steady-state voltage, conveyed on chip-wide signal to control the sensitivity. Transient voltages having a duration less than 1 nanosecond can be sensed, and the amplitude threshold can range from tens of millivolts to volts.
One aspect of the present invention is generally defined as a method for detecting a missing or defective power supply pin connection or other defect that results in excessive transient voltages on power supply rails of a circuit having, for each of one or more power rails, a stimulus logic gate connected to the power rails, a sensor logic gate connected to the power rails and a memory element for storing the output of the sensor logic gate, the method comprising initializing the sensor logic gate to a dc voltage whose level is substantially independent of transient voltages on the power rails and initializing the memory element to a predetermined value; applying a signal transition to the stimulus logic gate connected to the supply voltage rail to cause a transient voltage in the power rails; and inspecting the output of the memory element for a change in state indicating that an excessive voltage transient was detected on the power rail.
Another aspect of the present invention is generally defined as a circuit for testing the quality of dc power supply voltage rails, the circuit comprising: a stimulus logic gate whose power supply is the supply voltage rails and whose changes of state causes a transient current in the supply voltage rails; a sensor circuit whose power supply is the supply voltage rails, and whose input is connected to a dc voltage reference whose voltage is substantially independent of transient voltages on the supply voltage rails; a latching circuit having an input for the output of the sensor logic gate; wherein a stimulus signal transition applied to the stimulus logic gate causes a change of state of the stimulus logic gate and the output of the latching circuit changes state if the amplitude of the transient voltage on the supply voltage rails exceeds a threshold.